The present invention relates to a data highway system with dual transmitting loop lines, and more particularly to a data highway system capable of returning to a normal operation by automatically forming a loop back pertinent to a break or disconnect of transmitting lines. Specifically, the present invention relates to a data higway system comprising a plurality of data stations connected through dual transmitting loop lines, and detecting means for detecting the break or disconnect of lines incorporated with each data station, thereby automatically detecting the break or disconnect of transmitting lines constituting a loop back in accordance with the line break.
There have been proposed various kinds of data highway systems for transmitting data between data stations connected by loop transmitting lines. These data highway systems are suitable for various purpose, e.g. remote control of an electric power system including a large number of data stations. A well known transmitting system comprises a master station, and a plurality of slave stations independently connected to the master station by transmitting lines, a data highway system makes it possible to reduce the number of transmitting lines, thereby to provide a low cost system at a low cost. However, if the loop transmitting lines are broken or disconnected due to any trouble in the system, it is impossible to transmit the data, with the result that the system is down.
Attempts have been made to eliminate these troubles which have been encountered with the data highway systems.
For instance, there has been proposed a data highway system with dual transmitting lines in combination with a master station and a plurality of slave stations, wherein a break or disconnect line detecting command transmitted from the master station is scans between the slave stations; such a system is known as a "polling" technique. When a line break or disconnect occurs, the master station transmits a loop back command to the slave station via the broken or disconnected lines on the basis of the polling result. Connections are established to connect the first transmitting line to the second transmitting line so as to constitute a loop back to return to normal condition. However, with this method, it is impossible to have real time processing. Accordingly, it is difficult to apply this method to certain systems, e.g. an electric power system, where it is not permissible to temporarily interrupt the power.
According to another prior art system, and "echo-back" method, such as, in a telephone system, is employed. This method is characterized by a synchronizing bit in a data format. The synchronizing bit is employed as an echo-back command. This method makes it possible to test terminal equipments by the echo-back command to determine whether each terminal equipment normally operates. However, this prior art system is not provided with the means for returning to normal condition. Accordingly, this prior art is not suitable for repairing the line breakage in an electric power system.
In a telephone system, similar to the last mentioned prior art, when line breakage or disconnection is detected, the concerned terminal equipment connects the first transmitting line to the second transmitting line so that a loop back is formed and an alarm signal. The alarm signal is transmitted to the adjacent terminal equipment. The terminal equipment including a breakage line receives the alarm signal via a transmitting line that is loop-backed to recognize that the loop-back is established, thereby ceasing to produce the alarm signal. With this method, there is drawback that data derived while the loop-back is being formed is disregarded.